Loop flows in a ring AC power system

The interconnection of AC power systems surrounding a sea or a chain of mountains may lead to a structure having a ring shape. An example is the projected Mediterranean ring, planned to be operational within a few years. In the past, large circulating power flows of about 1000 MW were recorded in the ring power systems around the lake Erie and around the Rocky Mountains in North America. This paper shows that a stable power flow may circulate in a ring AC power system and corresponds to a total phase shift of 2pi (or a multiple of 2pi) radians along the ring. The various power flow solutions of a theoretical system are shown as a function of the power transfer between two areas. Simulations of the system dynamic behaviour show the presence of a loop flow due to a line tripping. The reclosure of the tripped line does not always restore the original power flow. The easiest way to get rid of it is to increase the equivalent impedance of a well-chosen path of the ring power system by opening one or several tie lines. The subsequent reclosure of these lines keeps the system in the normal state. (C) 2003 Elsevier Science Ltd. All rights reserved.